12)

$$12) (\frac{1}{3} - \frac{1}{14}) : \frac{3}{7} = (\frac{1 \cdot 14}{3 \cdot 14} - \frac{1 \cdot 3}{14 \cdot 3}) : \frac{3}{7} = (\frac{14}{42} - \frac{3}{42}) : \frac{3}{7} = \frac{11}{42} : \frac{3}{7} = \frac{11}{42} \cdot \frac{7}{3} = \frac{11 \cdot 7}{42 \cdot 3} = \frac{11 \cdot 7}{6 \cdot 7 \cdot 3} = \frac{11}{6 \cdot 3} = \frac{11}{18}$$

Ответ: $$\frac{11}{18}$$